PLANT PHYSIOLOGY

Plant stress

Plants encounter many enemies that cause them stress! Fortunately, they have developed systems of resistance to combat those attacks that are greatly feared in farming: frost, drought, disease, insect and animal enemies of crops, etc.

Resources are available, such as elicitors, to help them resist and in so doing keep treatments to a minimum.

Biotic stress: what is it?

Firstly, there is biotic stress: Triggered by fungi, insects, bacteria and weeds.

Some history: As long ago as 1961, Ross showed that by inoculating a tomato leaf with a mosaic virus strain (TMV), there was increased resistance by the other leaves to this virus as well as to other pathogens. Could plants have an immune system?

Subsequently, cellular and molecular biology advances have demonstrated that resistance is acquired or induced. Naturally less developed than animal immune systems, plants do in fact possess systems of resistance to control aggression. Plants “recognize” microorganisms by means of “signalling” molecules integrated in plant cell walls. Some microorganisms are beneficial and symbiotic (mycorrhizae, rhizobium, etc.). Others are pathogenic and cause disease (powdery mildew, mildew, botrytis, fusarium, pythium, rhizoctonia, etc.). When a pathogen “attacks” a plant, the latter will set in motion a series of defence reactions within the cell.

How do plants respond?

by “cellular suicide”: the plant sacrifices cells on the site of the infection to halt the pathogen

by strengthening the mechanical barrier through a thickening of the cell wall

by the production of metabolites having an antimicrobial activity, especially phytoalexins (Langcake and Pryce, 1976)

by the production of enzymes that cause deterioration of pathogen walls such as glucanase and chitinase (Van Loon, 1997).

It should be underlined that the “immune response” is systemic in character (throughout the plant) and no longer local (like the cell suicide). This so-called SAR (Systemic Acquired Resistance) is based on the activation of genes that maintain the whole plant in a state of resistance against a wide array of pathogens.

Elicitors: a solution against diseases?

SPD’s (Stimulators of Plant Defences) or elicitors can “mimic” pathogen attacks to prepare plants for an actual arrival of the disease.

A large number of agents can provoke a reaction in the plants without, however, causing the disease. They involve, more often than not, microbial extracts, plant extracts, organic compounds, minerals and physical agents. They are recognized by the plant’s membrane receptors, in the same way as an actual pathogen and enable their preparation for greater subsequent resistance to disease.

An elicitor (SPD) is a product whose function is to trigger the plant’s defence system sufficiently early to prevent the disease from developing. Their efficacy is therefore only preventive. Once the pathogen has been introduced it will be difficult to remove it. Direct actions could be considered necessary (insecticides, fungicides, etc.).

Elicitors (SPD’s) are elements that should be taken into account if rational farming methods are sought. However, they do sometimes enable treatments to be kept to a minimum. But they remain complementary to conventional control strategies.

In any plot of land, each individual plant is a prey for the various very numerous aggressors. We have seen that plants possess an immune system enabling them to resist the diseases that are so feared in agriculture: mildew, powdery mildew, fusarium, etc. Means are however available to assist the plant to defend itself against abiotic stress: Such as osmolytes.

Abiotic Stress: what is it?

Abiotic stress is triggered by:

frost:

heat,

thermal shock,

salinity,

lack of water,

solar radiation,

nutritional deficiencies,

wind or lodging, etc (refer also to other stresses).

How do plants react to sudden changes in temperature?

Temperature changes can cause leaf necroses such as “tip burn” in lettuce; variations of the water regime, the “black rot” disease in tomatoes.

Which mechanism do plants use to survive nutritional deficiencies?

They slow down their metabolism and reduce their use of energy. That in turn reduces growth, photosynthesis and therefore yields!

Some plants defend themselves better than others to extreme conditions of aridity, low temperatures or salinity.

For instance, algae live in ultra-saline environments (salt is toxic for plants), some species like the rose of Jericho survive decades without water in the desert, etc.

Re-hydration of a rose of Jericho

The rose of Jericho can survive without water for several years. It unfolds once more (actual time: 3 hours) when it is in a moist environment. The leaves gradually become green again (wikipedia). Plants have the genetic capacity of synthesizing protective substances such as “osmolytes”.

Glycine betaine is the most powerful osmotic protector in the plant world. It increases the osmotic pressure in the plant cell to prevent water leaking from the cell leading to its death.

It enables the retention or diffusion of water and trace elements by means of managing this osmotic pressure. In the same way, it diminishes the point of crystallization of water inside plant cells; this enables protection from frost thus avoiding the bursting of the cells and the plant’s death.

In wine growing, for instance, when applied to leaves at the end of flowering, betaine glycine improves nutrition in the event of heat waves. And 3 weeks before harvest, it enables improved resistance to bursting / cracking of berries and better resistance of post harvest fruits (table grapes). It has been shown that it enables enhanced conservation for fruits after harvest (Momilia). Success as a whole is due to anticipation of attacks. All farmers are aware of the risks associated with high temperatures (weather forecasts are vital..) and they know when frost can still occur in the Spring. Osmotic protectors have a bright future in farming to keep weather damage to the environment to a minimum